Bacteria within the order Rickettsiales would have little impact on human and veterinary medicine in the absence of the arthropod vector. Interestingly, the influence of primary infections with one Rickettsia sp. can influence the success of transovarial transmission of a second. This chapter details some fascinating trends observed regarding vertical and horizontal transmission. Biotic and abiotic factors determine the stability of any sylvatic or zoonotic transmission cycle. The chapter centers on a discussion of the attributes of successful pathogen transmission in the context of the vector's ability to modulate (i) the mammalian host's response during acquisition and transmission and (ii) microbial growth within the vector during the maintenance phase. The discussion in these two sections essentially defines the environment and competency of both the vector and mammalian host as determinants of transmission and transmission efficiency. The chapter ends with a survey of fluctuating ecological trends that can enhance or diminish the potency of vector-borne rickettsial zoonotic cycles. Even though acquisition rates were similar for each transmission experiment, intergenera transmission required cofeeding of multiple infected mites with uninfected mites. Rickettsial diseases have the potential to change the outcomes of war and prey on the unfortunate circumstances that arise from disaster.